"There are two reasons high altitude leaves players lacking air," club doctor Santiago Calvo told local radio station Sol 91.5. "On the one side, there is less oxygen.

"For this, we have oxygen tubes to be used before the match, at half-time and after the game. The other side is the pressure from the atmosphere. This increased pressure creates a vasoconstriction; the arteries become smaller around the lungs.

"This decreases the interchange of oxygen and carbon dioxide -- and that is where the Viagra comes in. It widens the blood vessels around the lungs."

Calvo added: "No player is obligated to take [the drug] -- but it is not a banned medication."